Computer System
Lecture 8, hour 3
Nov.3

Permission / Protection
-----------------------

File Protection: who X what
e.g. "Mike can delete"

What: Read, Write, Execute, Delete, Append, List, CD(change directory)

Unix-like
                File                        Directory
              -------                      -----------
Read            Yes                            List
Write       Write, Delete, Append         Append Files
eXecute        program                          CD

Above are all Unix supported.

The most common:   ACL (Access Control List)


Who in Unix:
   user:  owner of file or directory
   group: Unix notion of sets of users defined by the system
   other/universe: all users

Q: How are they represented?

 -   rwx   r--   r--
---  ---   ---   ---
file user group other

e.g. 660: owner/group can read and write the file, no others.

*****************************************************************************

Q: How to implement between high level interface and low interface?
A: Divide the file systems.


Top layer
                    User Programs                     File system implementation
                   ---------------                  -----------------------------
metadata ---->  logical file system (VFS)  ---->     metadata
data     ---->  file-organization module   ---->     talking about files
                                                     maps logical addresses to disc blocks
                basic file system ----> queuing ----> only talking about blocks
                I/O cotroller              ---->     write_block / read_block 
                Devices (disc)
Low layer


Q: Why?
A: I/O controller depends on devices.
   For diversity of file system and flexibility.

High-level presentation of file system would be the same, which in Linux is called VFS 
that doesn't depend on underlying file system.

*********************************************************************************

Basic Data Structure
--------------------

Per-partition (volume)

    - Boot control block
        Information neccessory to boot an OS from this partition

    - Volume control block
        Information about layout of the volume
           - location of root directory
	   - file system type
	   - list of free (empty) blocks

Per-file

    - File control block  (FCB)
         file metadata
	 permission
         location of data
         access

In kernel memory
	mount tables
	open file tables
	buffer and cache



************************************************************************************

Example Operations
------------------

Open (_filename)
	- check if it's opened by other process
	- look in system-wide open file tests
	- if not there
		- find FCB in the file system
		- create entry in system-wide table
		- create entry in per-process open file table


Close (_filename)
	- delete per-process entry
	- delete the system-wide info